The invention concerns a swirling-flow burner comprising a burner tube and an oxidizer supply tube concentric with and spaced from the burner tube, defining an annular fuel gas conduit between the tubes, the oxidizer supply tube and the fuel gas conduit having separate inlet ends and separate outlet ends, a fuel gas injector connected to the outlet end of the fuel gas conduit, which fuel gas injector having a substantially U-shaped cross section around a common axis of symmetry of the burner tube and the injector and having an annular outlet directed inwardly towards the axis and a gas mixing zone, an oxidizer injector connected to the outlet end of the oxidizer supply tube and having an axis of symmetry common with the fuel gas injector, wherein the outlet of the oxidizer injector is directed towards the gas mixing zone, further comprising static swirler means provided in the oxidizer injector for producing a swirling movement of the oxidizer around the axis before the oxidizer is discharged from the oxidizer injector.
A burner for use in gas-fired catalytic reactors is disclosed in EP B1 545,440. The main field of application of the known burner is the production of hydrogen and carbon monoxide process gas by primary and secondary steam reforming or by autothermal catalytic reforming of a hydrocarbon fuel, where the problem is to produce a process gas with a low carbon soot content as the process has to take place with a substoichiometric oxidizer-supply. According to the prior art, this is counteracted by providing multiple mixing points for the fuel gas and the oxidizer by generating a central swirling flow of oxidizer that is guided to the mixing zone, where the fuel is supplied peripherally to the oxidizer flow. The swirling or rotating movement of the oxidizer is generated by pitched blades in the swirler.
Furthermore, as the combustion temperature is high, often above 1000.degree. C., there is a risk for overheating of the burner surface close to the combustion zone, when designing the burner for recirculation of hot gases for mixing purposes towards the burner surfaces. According to EP B1 545,440, the design of the burner directs the flow of combustion products away from the burner along the central axis causing the recirculation of the combustion products to take place at the cooler periphery of the combustion zone, whereby the gases are cooled before they reach the burner face and are mixed with and reheated by the central flow away from the burner.
As steam is usually supplied to the hydrocarbon fuel for moderating the flame temperature and enhancing hydrocarbon conversion, this also has the effect of suppressing soot formation. The swirler incorporated in the burner according to the known art has a mixing effect depending on the pitch angle of the blades of the swirler, and experiments have shown that an increasing pitch angle makes it possible to feed with a lower steam to hydrocarbon ratio, hereafter referred to as S/C ratio. While a blade angle of e.g. 30.degree. results in a soot limit at a S/C ratio of 0.9, a blade angle of 60.degree. lowers the soot limit at a S/C ratio of 0.6. It is desirable to lower the soot limit further, but physical conditions limit the blade pitch angles to about 60 to 75.degree..